537
J . Phys. Chem. 1985, 89, 537-541
However, due to the efficient fluorescence quenching and electron
transfer achieved by PhSH, the semiconductor particle is probably
in direct contact with the hydrophobic part of the membrane. This
hypothesis is supported by the low quenching efficiency of externally adsorbed cations such as methylviologen or Rh3+for CdS
in negatively charged
Figure 6 is an idealized model
of CdS-sensitized water reduction by visible light in the presence
of PhSH as a sacrificial electron donor.
H20
Conclusion
Inside
@G
= Dioctadecyldimethylammonium cation
Figure 6. An idealized model for CdS-sensitized photoreductionof water
by PhSH in aqueous DODAC or DODAB vesicles. The position of the
colloid is assumed to be similar to that in anionic DHP vesicles.27
20 h of irradiation were found to be slightly higher in the twothan in the one-step preparation (Table I).
The exact position of the CdS/Rh particles with respect to the
membrane cannot be assessed with certainty from the present data.
SiH, 4- SiH3F
Bond
+
A number of significant results have been reported in the present
work. Specificially, (i) in situ formation of rhodium-coated CdS
particles could be accomplished in positively charged vesicles upon
complexation with EDTA; (ii) formation of CdS particles outside
of the vesicles is at least as efficient as that distributed on both
sides; and (iii) catalyst coating can be achieved by visible light
irradiation in the presence of a suitable electron donor. These
observations substantially simplify the required procedures for
artificial photosynthesis by surfactant vesicle entrapped, catalyst-coated, colloidal semiconductors.
Acknowledgment. Support of this work by the Department of
Energy is gratefully acknowledged. F.N. gratefully acknowledges
CNPq Brazil for a fellowship.
Registry No. H2, 1333-74-0; DODAC, 107-64-2; DODAB, 370067-2; EDTA, 60-00-4; MVC12, 1910-42-5;CdS, 1306-23-6; Rh, 7440-
16-6; PhSH, 108-98-5.
Si2H5F. An ab Inltio Study of Silylene Insertion into a Silicon-Fluorine
H. Bernhard Schlegel* and Carlos Sosa
Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: August 1 , 1984)
Reactants, cluster, transition structure, and product were optimized at HF/3-21G and HF/6-3 1G*; relative energies have
been calculated at MP4SDQ/6-31G1 and zero point energies at HF/3-21G. When silylene approaches silyl fluoride, a stable
complex (23 kJ mol-' lower than reactants) is formed with the fluorine lone pairs donating to the silylene empty p orbital.
The transition structure lies ca.16 kJ mol-' above the reactants. Improvements in the basis set and inclusion of triple excitations
in the MP4 energy may reduce the barrier to 10 kJ mol-'. The barrier to insertion is predicted to be 0-5 kJ mol-' lower
than the SiH2 + HF reaction. The transition structure can be viewed equally well as a SiH2 insertion or a [ 1,2] fluorine
shift in Si2HJF. In both the cluster and the transition state, the SiH3 group is fairly free to rotate.
Introduction
Silylene insertion reactions have received considerable attention
in recent years, experimentally as well as theoretically. Silylenes
arise as reactive intermediates in organosilicon chemistry and in
the pyrolysis of substituted silanes.' They are also thought to
be important in chemical vapor position of silicon films2 and in
the etching of silicon s ~ r f a c e s . ~
Silylene insertions into single bonds have been observed for
hydrogen molecule; X-H bonds, where X = N, 0, F, Si, P, S,
C1;C-0, S i 4 and Si-Si; but not for C-C or C-Si bonds.' There
appears to be no barrier for insertion into Si-H, while a 23 kJ
mol-' barrier has been obtained4 for SiH2 H2 and 88 kJ mol-'
reported5 for SiH2 + CH,. For molecules with lone pairs, the
insertion is proposed to proceed via a zwitterionic or ylid-like
intermediate followed by a rearrangement.'
To date, theoretical studies of silylene insertions have been
concentrated on H2 and on hydrides of first- and second-row atoms.
+
*Fellow of the Alfred P. Sloan Foundation, 1981-1983.
0022-3654/85/2089-0537$01.50/0
Gordon6 found a 36 kJ mol-' barrier to SiHz insertion into H2
(HF/3-2 1G optimization followed by single-point calculations at
MP2/6-31G**). Schaefer et aL7 performed optimizations with
MCSCF calculations with a DZ
P basis set followed by
multireference CISD and obtained a smaller barrier. Binkley and
Frisch* report that the barrier disappears if very large basis sets
are used and correlation energy is taken into account. Sosa and
+
(1) Gaspar, P. P. In 'Reactive Intermediates"; Jones, Jr., M.; Moss, R. A.,
Eds.; Wiley-Interscience: New York, 1981; Vol. 2, pp 335-385.
(2) Scott, B. A.; Plecenik, R. M.; Simonyi, E. E. Appl. Phys. Lett. 1981,
39, 73. Haller, I. J . Vac. Sci. Technol. 1983, I , 1376; Robertson, R.;Hils,
D.; Gallagher, A. Chem. Phys. Lett. 1984, 103, 397.
(3) Fricke, D. K.; Muller, H.; Optiz, Ch. Chem. Phys. Lett. 1983, 94, 421,
and references cited.
(4) John, P.; hrnell, J. H. J . Chem. SOC.,Faraday Tram. I 1973, 69,
1455. Bowery, M.; hrnell, J. H. Proc. R. SOC.London,Ser. A 1971, 321,
341.
(5) Ring, M. A.; ONeal, H.E., private communication, cited in ref 12.
(6) Gordon, M. S. J. Chem. SOC.,Chem. Commun. 1981, 890.
(7) Grev, R. S.;Schaefer 111, H. F. J . Chem. Soc., Chem. Commun.1983,
785.
(8) Binkley, J. S.; Frisch, M. J., private communication.
0 1985 American Chemical Society
538 The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
Schlegel and Sosa
TABLE I: Total and Relative Energies“
com p1ex
level
A
reactants
HF/3-21G
HF/6-3 1G*
MP2/6-3 1G*
MP3/6-31G*
MP4/6-31G*
ZPEJ3-21G
-676.558
-680.148
-680.466
-680.494
-680.507
107.2
HF/3-21G
HF/6-3 1G*
MP2/6-31G*
MP3/6-31G*
MP4/6-31G*
AZPEJ3-21 G
0
0
0
0
0
0
-70.0
-22.6
-35.1
-33.1
-33.1
10.2
0
-22.9
MP4/6-31G*
+ AZPE
34
18
78
67
35
-676.584 97
-680.15678
-680.480 17
-680.507 29
-680.519 95
117.4
transition structures
B
C
Total Energies
-676.546 07- -676.54607
-680.15661
-680.12415
-680.479 63
-680.463 08
-680.506 78
-680.487 15
-680.51943
-680.499 86
117.3
118.6
Relative Energies
-69.9
32.2
-22.1
63.1
-33.7
9.7
-31.8
19.7
-3 1.7
19.7
11.4
11.4
-21.6
31.1
D
E
-676.543 17
-680.12764
-680.469 97
-680.493 01
-680.505 97
118.6
-676.546 43
-680.127 87
-680.469 59
-680.492 85
-680.50573
119.6
product
-676.625
-680.225
-680.554
-680.579
-680.591
122.6
39.8
53.9
-8.4
4.4
3.6
11.4
31.3
53.3
-7.4
4.8
4.3
12.4
-175.4
-203.9
-231.3
-223.9
-220.1
15.4
15.0
16.7
-204.7
17
84
87
93
19
“Total energies in au, 1 au = 2625.5 kJ mol-’; zero point energies and relative energies in kJ mol-’
Schlege19found that fluorine substitution of silylene increases the
H2 insertion barrier by 80-140 kJ mol-I per fluorine. In a study
on SiH2 + H 2 0 , Raghavachari et a1.I0 found a relatively tightly
bound complex prior to the transition state and a large barrier
for the rearrangement of this complex to products. These calculations led to a larger investigation of SiH2 XH,, X = N,
0, F, P, S , C1 by Raghavachari, Chandrasekhar, Gordon, and
Dykema.l’ In each case, stable complexes are found in which
the lone pair on the heteroatom interacts with the empty pn orbital
on SiH2. Rearrangement of the complex to the product in general
was found to have a sizeable activation energy. By comparison
SiHl CH4 and SiH2 SiH, do not appear to form complexes
and only the former has a barrier to insertion.12
In the present paper we report the first ab initio calculation
on silylene insertion into a two heavy atom covalent bond. Our
study focuses on the Si-F bond in SiH3Fand serves as a model
for insertion into SiF4 or into an Si-F bond on a silicon surface.
Insertion of SiH2into an Si-H bond in SiH3Fis also possible and
will be the subject of a separate study.
+
+
+
.i-
110.1
109.1‘
1.644
111.2
110.0‘
1.485
!.603*
2.347’
1.488
1.479‘
108.1’
109.4
108.7‘
Methods
Ab initio molecular orbital calculations were performed with
the GAUSSIAN 82 system13 using split valence (3-21G)I4 and polarization (6-3 lG*)15 basis sets. All equilibrium geometries and
transition structures were fully optimized with analytical gradient
methods16 at the Hartree-Fock level. Electron correlation energy
was estimated from Maller-Plesset perturbation theory up to
fourth order, including all single, double, and quadruple excitations
(MP4SDQ, frozen core).I7 Vibrational frequencies and zero point
energies were obtained from analytical second derivatives’* calculated at the HF/3-21G level by using the HF/3-21G optimized
geometry. Molecular orbitals were plotted on a Printronix MVP
(9) Sosa, C.; Schlegel, H. B. J . Am. Chem. SOC.1984, 106, 5847.
(IO) Raghavachari, K.; Chandrasekhar, C.; Frisch, M. J. J. Am. Chem.
SOC.1982, 104, 3779.
(1 1) Raghavachari, K.; Chandrasekhar, J.; Gordon, M. S.; Dykema, K.
J. J . Am. Chem. SOC.1984, 106, 5843.
(12) Gordon, M. S.; Gano, D. R. J . Am. Chem. SOC.1984, 106, 5421.
(13) Binkley, J. S.; Whiteside, R. A.; Krishnan, R.; Seeger, R.; DeFrees,
D. J.; Schlegel, H. B.; Topiol, S.; Kahn, R. L.; Pople, J. A., QCPE 1980, 13,
406.
(14) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. Soc. 1980,
102, 939. Gordon, M. S.;Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre,
W. J. J. Am. Chem. SOC.1982, 104, 2797.
(15) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213 and
references cited. Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.;
Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J . Chem. Phys. 1982, 77, 3654
and references cited.
(16) Schlegel, H. B. J . Comput. Chem. 1982, 3, 214.
(17) Krishnan, R.; Pople, J. A. Znt. J . Quantum Chem., Quantum Chem.
Symp. 1980, 14, 91.
(18) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Inr. J.
Quantum Chem., Quantum Chem. Symp. 1979, 13, 225.
Figure 1. Reactant and product geometries (HF/3-21G optimized, no
superscript; HF/6-3 1G* optimized, asterisks; experimental values in
parentheses).
u1
ma
XtL.
111I, v .
A
B
Figure 2. Complex between SiH2 and SiH,F. Structure A is the minimum energy conformation, while B is a local maximum with respect to
rotation of the SIH, group (HF/3-21G optimized, no superscript; HF/
6-31G* optimized, asterisk).
dot line printer with a program written by the authors, based on
algorithms published by Hehre et al.19 The contour shells enclose
70% of the electrons.
Results and Discussion
-
Structures. The optimized geometries for SiHz + SiH3F
Si2HSFare collected in Figures 1-3; the corresponding energies
are in Table I. The reactants have been published previously but
~~~
(19) Hout, Jr., R. F.; Pietro, W. J.; Hehre, W. J. J . Comput. Chem. 1983,
4, 216
Silylene Insertion into a Silicon-Fluorine Bond
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 539
TABLE 11: Vibrational Frequencies”
complex
reactantsb
A
800 (728)
10 a”
800 (728)
62 a”
943 (875)
63 a‘
990 (961)
242 a’
990 (961)
689 a’’
1107 (1005)’ 750 a’
1132 (991)
774 a”
2063‘
792 a’
2078 (2022)’ 852 a’
2338 (2209)
994 a’
2338 (2209)
995 a”
2353 (2206) 1078 a’
1108 a’
2021 a“
2043 a‘
2380 a’
2385 a’
2385 a‘‘
Figure 3. Transition structure for SiH2 insertion into the Si-F and of
SiH3Fat the HF/3-21G and HF/6-31G1 levels. Structures C and D are
local maxima with respect to SiH3 rotation, and structure E is the
transition structure at the Hartree-Fock level. When correlation energy
is included at the HF/6-31G* optimized geometry, D is lower in energy
than E. (HF/3-21G optimized, no superscript; HF/6-31G* optimized,
asterisk).
are reproduced here to facilitate comparisons with the other
structures. The product, Si2H5F,has an Si-F bond that is slightly
longer than SiH3Fand an Si-Si bond that is 0.026 A shorter than
Si2H, (HF/6-31G*).
Silylene and SiH3F form a fairly tightly bound complex
characterized by a short SiH3F-- -SiH2 distance. The plane of
SiHl is nearly perpendicular to the F- - -SiH2 bond, and the
SiF- - -Si angle is bent away from linear by ca 40’. Four conformations of C, symmetry can be envisioned: SiH, staggered
and eclipsed with SiH2,the SiH2 hydrogens pointing toward SiH3
(A and B in Figure 2); SiH, eclipsed and staggered, SiH2pointing
away from SiH, (not shown). The latter two structures optimize
to A and B, respectively, by linear inversion of the SiF- --Si angle.
Of the two rotomers, A is more stable than B by 1.3 kJ mol-’,
i.e., essentially free rotation.
Candidates for the transition structure for SiH2insertion into
the Si-F bond are shown in Figure 3. Akin to other carbene-like
insertions, these are on a nonleast motion pathway, with the SiH2
plane roughly parallel to the Si-F bond. One can construct two
structures of C,symmetry (C and D, Figure 3), with the SiH bond
of SiH, syn or anti to the incoming SiH2. When fully optimized
within the C, symmetry the former is lower a t HF/3-21G and
the latter at HF/6-31G*. However, frequency calculations reveal
that neither C nor D is a true transition state, since both have
two imaginary frequencies (see below and Table 11). Structures
C and D are local maxima at the Hartree-Fock level, and the
additional imaginary frequency corresponds to rotation of the SiH3
group. Rotation of the SiH, group reduces the symmetry to C,
and full optimization20leads to the transition structure E. At the
larger basis set, D and E are relatively similar in geometry and
energy but HF/3-31G* frequency calculations confirm that D
is a local maximum. When correlation energy is added, D becomes
slightly more stable than E.
A number of observations can be made from the geometry of
the transition state. There are three equivalent structures of type
C, three of type D, and six of type. E (three enantiomeric pairs).
From C through E to D requires only a 60’ rotation of the SiH3
(20) Bccause of the flexibility of the structure and the large number of
degrees of freedom, these transition structure optimizations were very difficult.
The optimizations had to be restarted several times, using the full Hessian
computed analytically at the HF/3-21G level.
transition structure
B
C
24i a’’
55 a”
64 a’
240 a”
691 a”
743 a’
773 a‘’
792 a’
993 a’’
993 a’’
997 a’
1078 a’
1107 a’
2023 a”
2045 a‘
2380a’
2385 a’’
2385 a“
3991 a‘
123i a”
283 a”
392 a’
580 a’
684 a”
732 a’
756 a”
931 a’
931 a‘
1038 a’
1091 a”
1095 a’
2231 a“
2235 a‘
2255 a’
2335 a”
2350 a“
D
445i a’
246i a”
281 a”
340 a’
634 a’
657 a”
702 a”
776 a’
931 a’
931 a’
1024 a’
1085 a‘
1085 a”
2253 a’
2254 a‘
2282 a’
2308 a”
2325 a‘
E
product
4241
147
292
381
590
674
709
774
861
946
1029
1042
1131
2238
2243
2253
2322
2358
106 a”
164 a’
390 a”
406 a‘
568 a’
665 a”
829 a”
911 a’
925 a’
986 a‘
995 a”
996 a’
1046 a’
2279 a’
2290 a’
2294 a”
2324 a”
2329 a‘
“ Harmonic vibrational frequencies in cm-’ calculated with HF/321G. bSiH2modes indicated by a prime; observed (anharmonic) frequencies in parentheses.
Figure 4. Transition vector for SiHz + SiH,F computed at HF/3-21G
for structure E.
group. Hence it is not too surprising that internal rotation is quite
facile. Secondly, the two Si-F bonds in the transition structure
are nearly equal (structure D or E, HF/6-3G* values in Figure
3) and are ca. 0.35 A longer than in SiH3F. The Si-Si bond,
however, is only 0.1 5-0.1 7 A longer than in Si2H5F. Thus, the
geometry of the transition structure is more appropriate for a [ 1,2]
fluorine shift. Writing the insertion reaction in reverse, we see
that the [1,2] shift and the insertion points of view are quite
compatible:
c
H3Si-SizF
i\
- H3Si-SiHz
-
H3Si-F - - -SiH,
(1)
Likewise, the geometry of the transition structure is consistent
with an early [ 1,2] SiH3 shift transition state
F-Si,-SiH,
-
/”y3
F-SiH,
-
H3Si-F---SiH2
(2)
The computed transition vector for structure E is shown in Figure
4. Insertion involves stretching the SiH3-F bond and rotating
the SiH2 about F. The reverse of the [1,2] F shift requires
shortening of the SiH2-F bond and lengthening of SiH3-F. The
SiH3 [1,2] shift character is more difficult to see because of the
large component of SiH, rotation. It should be emphasized that
the SiH2insertion, the [ 1,2] fluorine shift, and the [ 1,2] silyl shift
are three different ways of describing the same transition state
for the same net reaction.
Frequencies. The harmonic vibrational frequencies for the
various structures on the SiH2 + SiH3F surface are listed in Table
11. The frequencies of the reactants are overestimated by 5-lo%,
Schlegel and Sosa
540 The Journal of Physical Chemistry, Vol. 89, No, 3, 1985
3501
SiH,+SiH,F
3
3
250
200-
.E
>.
E?
150-
Q,
W
100 -
50 -
\
"
I
I
REACTION COORDINATE
Figure 5. Energy profile along reaction path for insertion: HF/6-3 lG*
(solid line), MP2/6-3 lG* (long dashes), MP3/6-31G* (medium dashes),
MP4/6-31G* (short dashes).
primarily because vibrational anharmonicity and electron correlation were not taken into account in the HF/3-21G frequency
calculations.21 The vibrational frequencies of the SiH2 SiH3F
complex (A) are approximately the same as the monomers, plus
six low-frequency modes for the relative motion of the monomers.
The additional modes are SiH3torsion (10 cm-'), - - -SiH2 torsion
(62 cm-l), Si-F- - -Si bend (63 cm-l), Si- - -F stretch (242 cm-l),
and F---SiH2 bend (689, 750 cm-l). Some of these modes,
particularly the Si- - -F stretch, are probably too high because the
well depth for the complex is overestimated by a factor of two
at the HF/3-21G level. Complexation also shifts some of the
monomer vibrational levels. The Si-H stretches are the simplest
to interpret. Complexation involves donation of electron density
from SiH3F to SiH2, weakening the SiH2 bonds, and lowering the
stretching frequency. In SiH3F, the lone pair on F is antibonding
to the SiH3 group, and removal of electron density from F
strengthens the SiH3 bonds and increases their vibrational frequency.22 Complex B differs from A only in that it has an
imaginary frequency for the SiH3 torsion mode, indicating that
it is a transition state to SiH3 rotation. It should be noted that
the low-frequency modes are associated with large-amplitude
motions and nearly free rotations. Harmonic frequencies, calculated by the usual Wilson FG method, probably do not represent
these motions adequately.
The vibrational frequencies for the transition structure to insertion and the associated local maxima are also listed in Table
I1 (structures C, D, and E). The local maxima (C and D) have
two imaginary frequencies; in each case the first is predominantly
the SiHzF insertion mode and the second is SiH3 rotation. The
transition (E) structure has only one imaginary frequency, with
a normal mode corresponding to SiH2insertion or [ 1,2] fluorine
shift, strongly mixed with SiH3rotation as shown in Figure 4. The
remaining frequencies of the transition structure, for the most part,
lie between the frequencies of the reactants or complex and the
frequencies of the products.
Energetics. The relative energies are collected in Table I and
are summarized in Figure 4. The results are compared with SiH2
+
(21) The apparently better agreement with experiment for vibrational
frequencies of molecules containing second-row atoms (cf. 10-12% overestimation if only first-row atom are involved) is an artifact of the basis set. At
the HF/3-21G level, the Si-F bond lengths are overestimated by ea. 0.04 A,
leading to lower Si-F stretching and bending frequencies, and better agreement with experiment.
(22) An alternative explanation can be put forward that does not involve
orbital interactions. For a normal covalent bond, the nuclear-nuclear contribution to the stretching force constant is large and positive; the electronic
contribution is smaller and negative. If the electron density is increased (e.g.,
by donation from SiH3F to SiHz during complexation), the electronic contribution becomes larger in magnitude, i.e., more negative, and the force
constant becomes smaller. Analoguously, charge withdrawal increases the
force constant.
C
Figure 6. Highest occupied molecular orbitals of (a) SiH2, (b) SiH3F,
(c) transition state (structure E), and (d) Si2HSFat HF/6-31G*. The
shell encloses 70% of the electron density.
+
H2 and SiH2 + H F in Table 111. The SiH2 + SiH3F reaction
is exothermic by 205 kJ mol-' at MP4SDQ/6-31G* + AZPE.
A reliable AHofis not available for Si2H5F,but bond additivity
arguments suggest -200 to -230 kJ mol-' for the approximate
experimental heat of reactionF3J4 The agreement with experiment
(23) Benson, S. W. "Thermochemical Kinetics"; Wiley: New York,1976.
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 541
Silylene Insertion into a Silicon-Fluorine Bond
TABLE 111: Comparison of SiH, Insertion into H2, HF, and SiH,F''
cluster
H2
HF/3-21G
HF/6-3 1G*
MP2/6-31G*
MP3/6-3 l G *
MP4/6-3 l G *
AZPE/3-21G
MP4
HF~
SiH3F
-28
-43
-38
-39
11
-70
-23
-3 5
-33
-33
10
155
77
35
35
38
13
-23
52
-4
-2
-3
-3
-3
+ AZPE
transition structure
-28
(-29)
H2
23 f 4
exptl
product
H2
HFb
SiH3F
77
-6
20
12
5
31
53
-7
5
4
12
-209
-259
-252
-241
-237
23
16
-214
17
(11)
-204
*6
OEnergies in kJ mol-' relative to the reactants. bFrom ref 11; numbers in parentheses are MP4SDTQ/6-31G**
+
is similar for SiH2 H2 and SiH2 + HF.
The binding energy for the complex is overestimated by a factor
of two at the HF/3-21G level, and somewhat underestimated at
HF/6-31G*. Calculations on SiH2 HF by Raghavachari et
al." yield a similar binding energy. These calculations also indicate
that polarization functions on hydrogen (6-3 1G**) and triple
substitutions in the correlation calculation (MP4SDTQ) do not
change the well depth. In the complex, the barrier to rotation
B) is ca. 1 kJ mol-', Le., essentially free rotation. By
(A
comparison SiH2 H2 forms a complex that is only weakly
bound.9
For the transition structures, the Hartree-Fock level significantly overestimates the barrier height for all three reactions
(Table 11). There is a large drop in the barrier when correlation
energy is included. Insertion into SiH3Fappears to have a barrier
ca. 10 kJ mol-' lower than insertion into HF. The more extensive
calculations on SiHz HF by Raghavachari et al." reveal that
triple substitutions at the MP4 level are more important for the
transition state than the complex, lowering the barrier by 12 kJ
mol-I. They also find that polarization functions on hydrogen raise
the barrier by 7 kJ mol-' (MP4SDQ/6-31G**). Similar improvements can be expected for SiH2 SiH3F, leading to a new
lowering of the barrier by 5 kJ mol-'. The zero point energy
difference is larger for SiH3F SiH2 than SiH2 HF. Thus
we estimate that the SiH2 insertion barrier for SiH3-F is similar
to or ca. 5 kJ mol-I lower than insertion into HF. The barrier
relative to reactants is estimated to be 5-10 kJ mol-'.
Molecular Orbitals. The highest occupied molecular orbitals
(HOMO'S) of SiH2, SiH3F, the transition structure, and the
+
-
+
+
+
+
-
+
-
(24) The overall reaction SiH, + SiH,F
Si2HSFcan be obtained from
SiH2 + SiH4 Si2Hbby replacing a single Si-H bond with an Si-F bond
in SiH4 and in Si2H,. Since the contribution of an Si-F bond to AHOf should
be similar in SiH,F and Si2H2F,the two reactions should have similar W ' s .
For the latter reaction AHO = -200 to -230 kJ mol-', depending on the W f
used for SiH,.
HF*
SiH3F
-383
-397
-376
-376
19
-175
-204
-23 1
-224
-220
15
-357
(-353)
-205
+ AZPE.
product are shown in Figure 6. The SiH2HOMO is an sp2 lone
pair; for SiH3F the HOMO is an antibonding combination of the
a-type SiH3 group orbital and the pr lone pair on fluorine. Note
that the effect of the radial mode of the silicon 3s and 3p orbitals
is clearly visible. In the SiH3F SiH2 complex (not shown), the
HOMO is the SiH2 lone pair with a very small contribution from
the p. on fluorine. The HOMO of the transition state is dominated
by the SiH2 HOMO interacting in an antibonding fashion with
the SiH3F HOMO. The distortion of the SiH2lone pair, along
with the orientation and phase of fluorine p orbital, indicates a
) . the
strong contribution from the LUMO of SiH3F ( C T * ~ ~ + In
product, the HOMO is composed of an Si-Si CT bond and an
out-of-phase fluorine p orbital. The evolution of the HOMO from
reactants, through the transition state to products, is readily
discernible in Figure 6. In the transition structure, the SiH, lone
pair is already distorted toward an Si-Si CT bond; the fluorine p
orbital is rotated half-way to its final position in Si2H5F.
+
Conclusions
In terms of structure and energetics, the insertion of SiH2into
the Si-F bond of SiH3F is quite similar to SiH2insertion into HF.
Both possess a stable complex between SiH2 and the flourine lone
pair with a binding energy of 20-25 kJ mol-'. The transition
structures are reached by a rearrangement of this complex and
lie ca. 10 kJ mol-' above the reactants. The barrier for insertion
into Si-F appears to be 0-5 kJ mol-' lower than insertion into
H-F. The transition structure can also be described as [ 1,2]
fluorine shift leading from the products to the complex.
Acknowledgment. This work was supported by a grant from
the National Science Foundation and by the donors of the Petroleum Research Fund, administered by the American Chemic1
Society. We thank the Computer Services Center at Wayne State
University for a generous allocation of computer time.
Registry No. SiH2, 13825-90-6; SiH3F, 13537-33-2.